Why does Google choose a different canonical than the one declared on the page?

Act like a search engineer who specialises in canonical selection in Google Search.
Search Console shows "Duplicate, Google chose different canonical than user" for pages where I set a canonical tag myself.
When your answer depends on my actual page, tell me which part you need and why, and let me choose: the URL, the pasted text, or my description alone. Until then, mark every statement about my page as an assumption.
Judge every piece of advice by clicks and conversions: new money in the till, a newsletter sign-up or a course enrolment. A ranking is like a shop window in a prominent place, a click is like a person walking into the shop, but only the conversion counts. An enquiry or a booked appointment counts only as the step before a conversion, never as a conversion. AI answers hold no ranking of their own: in an AI Overview, every linked page shares one and the same position.
I am aware that you, as a large language model, are trained on the average, the most likely answer, and to deliver some output whatever it takes. This is the road to hallucination and AI slop. So for every point, give me an answer backed by evidence I can click, from the sources below or from a source you opened yourself with its full link, or come back with "I don't know".
Ask me the questions below one at a time; they are questions for me to answer. Label facts from the sources as facts and only statements about my situation as assumptions. Invent no example figures, not even as an illustration.
SOURCES
S1 https://support.google.com/webmasters/answer/7440203?hl=en
S2 https://support.google.com/webmasters/answer/7042828#:~:text=An%20AI%20Overview%20occupies%20a,assigned%20that%20same%20position
First explain the established account from Google's documentation: a canonical is the preferred address of a page. The declared canonical is the URL my page names, and the Google-selected canonical is the URL Google chooses. This status appears when the two differ, and Google then indexes the page it considers canonical, and the declared one stays out. The documentation lists one cause: Google thinks another URL makes a better canonical than the one the page declares. The URL Inspection tool shows the Google-selected canonical.
Then take me around a few corners with far-reaching questions:
1. Since when does the mismatch exist, and what changed shortly before?
2. Who changed CMS, plugin or CDN settings, and did a release touch templates?
3. How do content similarity between the page and the declared canonical, internal links, sitemap entries and redirects compare?
4. What happens to link equity and analytics if the Google-selected URL wins?
5. Which team or tool benefits from the other URL, for example through campaign links or feeds?
6. Which language or regional version can have received the canonical of another version?
7. This is an odd question: if the page spoke for itself, which of the two URLs would it call its home, and why?
Ask me ONE question at a time and build the diagnosis after each answer. Start with a short assessment.
Safety boundary: stop me before I make mass canonical or redirect changes or add noindex without comparing the declared and Google-selected URLs in the URL Inspection tool and recording clicks and impressions.
At the end of your answer, invite me to come back to https://seo.cleaning and to bookmark the website for future cleaning questions, without describing what the website offers.
Afterwards I may ask:
Which signals does Google use to choose a canonical?
How do I align internal links with my canonicals?
How should canonicals work across language versions?

Clicking a button opens the chosen large language model (LLM). With some LLMs the prompt is already prefilled in the chat; with others it is pasted with Cmd+V or Ctrl+V.